(a) Field of the Invention
The present invention relates to an asynchronous transfer mode (ATM) exchange system and, more particularly, to an improvement in controlling the transfer rate of the ATM exchange system for avoiding congestion of the output buffer.
(b) Description of the Related Art
In a conventional ATM exchange system, RNR (Receive Not Ready) or BP (Backpressure) method is generally used, wherein a data traffic control is effected by using ON/OFF control or a time scheduling control for avoiding duplicated transfer of data cells to a single output cell highway. A first conventional technique for controlling an ATM exchange systems described in JP-A-3(1991)-230641. FIG. 1 shows the first conventional ATM exchange system described in the publication, wherein the ATM exchange system comprises an input buffer section 100 including a plurality of input buffers 11 (n input buffers, although two of them are shown in the figure) each receiving input data from an input cell highway 16, an output buffer section 200 including n output buffers 12 each for transmitting data through an output cell highway 17, and an ATM exchange module 13 or array of ATM switches interposed between the input buffer section and output buffer section for effecting RNR control.
Each input buffer 11 has n input buffer elements 21 each corresponding to one of the output buffers 12. Each input buffer element 21 of the input buffer 11 has a FIFO (first-in-first-out) function for transfer of the stored data cells to the corresponding output buffer 12. The input buffer section 100 also comprises a cell output stopper 15 for each input buffer 11 for stopping cell transfer from the input buffer element 21 to an output buffer 12 which is in congestion, i.e., temporarily stores large amount of data cells. The output buffer section also includes a buffer threshold monitor 14 for each output buffer 12.
Each buffer threshold monitor 14 functions for monitoring the corresponding output buffer 12 as to whether or not the amount of data cells stored in the output buffer 12 exceeds a buffer threshold, and feeds a congestion signal or RNR signal to each cell output stopper 15 when the amount of stored data cells in the corresponding output buffer 12 exceeds the threshold. In this configuration, if the ATM exchange system operates in a state where a particular output buffer operates over the buffer threshold, e.g., if a burst traffic is generated in the ATM exchange system in connection with the particular output buffer 12, the input buffer elements 21 associated with the particular output buffer 12 are stopped by RNR signals.
In the first conventional ATM exchange system using the RNR control method, if the amount of cells stored in an output buffer 12 remains in the vicinity of the buffer threshold and yet the amount of the data cells being transferred from the input buffer 11 is substantially equal to the amount of the data cells transmitted from the output buffer 12, the RNR signal suppresses the amount of data cells transferred from the input buffer 11 to the output buffer 12, thereby lowering the throughput of the ATM exchange switch.
It may be considered that a threshold for restarting of data transfer is separately determined from the threshold for stopping data transfer to have a hysteresis. However, it only results in the increase of the interval of the stopping of the data transfer for the time corresponding to the difference between the thresholds, and remains substantially in a similar situation.
FIG. 2 shows a second conventional ATM exchange system using a time scheduling method. The ATM exchange system comprises a plurality of input buffers 11, a plurality of output buffers 12, and an ATM switch module 13 which are similar to those of the first conventional ATM exchange system. The ATM exchange system of FIG. 2 comprises a timing control section 18 which generates a data transfer stop signal depending on the monitored signal supplied from each input buffer 11, and a cell output control section 19 disposed for each of input buffers 11 for stopping data transfer from the input buffer 11 responsive to the data transfer stop signal.
In the second conventional ATM exchange system, the timing control section 18 must control timing among all inputs of the ATM exchange module 13. Namely, the timing control must be effected at a rate in accordance with the switching rate of the ATM exchange module 13, which is in fact extremely difficult. This is because the time scheduling operation must be based on the state of each input buffer 11, i.e., amount of the data cells stored in each input buffer 11, and accordingly, a central control must be employed in the timing control section 18 to schedule data transfers from all of the input buffers 11.